The present invention relates to a semiconductor laser which is small in oscillation spectral line-width and variable in oscillation wavelength.
By reason of their small size, high efficiency and high reliability, semiconductor lasers have already been put to practical use as the light source for fiber optic communication. Systems heretofore employed utilize the direct modulation capability which is one of outstanding features of the semiconductor laser, and one of such conventional systems is what is called a direct intensity modulation-direct detection (DIM-DD) system in which intensity-modulated light corresponding to the amount of current injected into the semiconductor laser is received directly by a photodiode or avalanche photodiode after being propagated through an optical fiber. A dynamic single wavelength laser which stably operates at a single wavelength even during high-speed modulation, such as a distributed feedback (DFB) semiconductor laser, has been developed for use as the light source in the DIM-DD system, with a view to lessening the influence of dispersion of a single mode fiber so as to increase the repeater spacing.
On the other hand, it is possible to substantially improve the receiving sensitivity and hence increase the repeater spacing more than in the DIM-DD system, by positively utilizing the properties of the wave motion of light, such as its frequency and phase. This system is referred to as a coherent transmission system, which is being given much study experimentally as well as in its theoretical aspect and is now being regarded as a promising future optical communication system (see T. Okoshi, Journal of Lightwave Technology, Vol. LT-2, pp. 341-346, 1984, for example). In the coherent transmission system it is requisite, because of its property, that the light source at the transmitting side and the light source as a local oscillator at the receiving side be small in spectral line-width and variable in oscillation wavelength. In studies made so far on a laboratory scale, intended primarily for evaluating the potential of the system, it is customary to use a gas laser of an extremely small oscillation line width or more practical ordinary semiconductor laser in which an external diffraction grating is provided and light of only a specific wavelength is fed back thereto, thereby achieving high coherence and making the oscillation wavelength tunable. Since the light emitting region of the semiconductor laser is as small as about 1 .mu.m in diameter, however, the laser structure in which the light source and the external diffraction grating are not integrated is readily affected by mechanical vibrations and heat variations, unstable in providing desired characteristics and involves a large-scale system configuration; therefore, it is evident that such a laser structure is not suitable for practical use.
For the reduction of the oscillation line-width it is an effective method to increase the length of a resonator of the laser. In general, however, as the resonator becomes longer, the resonance wavelength spacing also becomes narrower correspondingly, leading to defects of liability to multi-wavelength oscillation and unstability of the narrow line-width characteristic. In addition, wavelength tuning is performed by selecting resonance wavelengths discontinuously, not continuously; accordingly, this semiconductor laser is not suitable for practical use.